1. Field of the Invention
The present invention relates to a digital filter apparatus suitable for use in an equalizer unit and a tone control unit for an audio signal and using an infinite impulse response digital filter (hereinafter sometimes referred to as an "IIR filter").
2. Description of the Related Art
Since a CD player to be mounted on a vehicle has a problem in that medium and bass tones cannot easily be heard because of road noise and/or engine noise, a filter apparatus, such as a tone controller or a loudness controller, is employed to make variable the output level of the audio signal in the medium and bass tones (about 200 Hz or lower) in order to emphasize the output level in the medium and bass tones to an appropriate level.
Since the inclination of the damping property of a conventional filter apparatus of the foregoing type is considerably moderate such as 6 dB/oct, the damped side tone adjacent to the medium and bass tones is adversely affected by the filter characteristics.